Changes between Version 15 and Version 16 of Tutorial/CorrelationImageVelocimetryOptimisation


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Timestamp:
Jan 28, 2015, 11:11:26 AM (6 years ago)
Author:
sommeria
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  • Tutorial/CorrelationImageVelocimetryOptimisation

    v15 v16  
    1818To observe the influence of the search box, come back to the GUI civ_input, set '''[!CorrBoxSize]'''=[19 13] and '''[!SearchBoxSize]'''=[27 25] with '''[Shift]'''=0, and visualise the result with '''uvmat'''.  Many black vectors (F=-2) are obtained, showing that the search domain  is too small, so that the correlation maximum is constrained by the  limited search interval. Using  '''[!TestCiv1]''' , it can be seen that the correlation maximum is indeed at the edge of the Search box in the main flow with u$\simeq$4 (while a gap of 2 pixels is required to properly determine the maximum without edge effect).
    1919
    20 The parameter '''[num_!CorrSmooth]''' is used to fit the correlation data with a Gaussian function to obtain the maximum with sub-pixel precision. We generally keep the default value 1, while the value 2 should be more appropriate for larger particles (with wider correlation maximum).
     20The parameter '''[num_!CorrSmooth]''' is used to fit the correlation data with a Gaussian function to obtain the maximum with sub-pixel precision. We generally keep the default value 1, while the value 2 should be more appropriate for larger particles (with wider correlation maximum). The quality of this feature can be tested by taking the image autocorrelation, selecting the option 'displacement' instead of 'PIV' in the menu '''[wiki:ListCompareMode "[!ListCompareMode]"]''' of '''civ_input'''. Then run the civ1 computation with '''series'''. Visualise the velocity field with uvmat: it is very close to 0 as expected but the histogram of the error can be estimated with the Tool/rectangle.  The curve exported from view_field is shown in figure **, comparing '''[wiki:CorrSmooth !CorrSmooth]='''1 and 2'''. '''We see that the histogram is somewhat more narrow for[https://servforge.legi.grenoble-inp.fr/projects/soft-uvmat/search?q=wiki%3ACorrSmooth !CorrSmooth]=1, corresponding to a slightly better result, but the typical error of the order of 0.1 px in both cases. ''''''
    2121
    2222The parameters '''[num_Dx]''' and '''[num_Dy] '''define the mesh of the measurement grid, in pixels. Reduce them to get more vectors, but keep in mind that the spatial resolution is anyway limited by the size of the correlation box, so that velocity vectors become redondant when the sub-images highly overlap those of the neighboring vector. Then the choice Dx=Dy=10, about half the correlation box, provides a good optimum.
     
    5151The parameter '''[deformation]''' (check box) improves the prior estimate by deforming the subimage taking into account the velocity gradients, so it can improve the processing in zones of strong shear or strong rotation, like vortex cores. It involves an interpolation of the sub-images to perform the deformation.
    5252
    53 Fix2 and Patch2 act on the Civ2 results like Fix1 and Patch1 on the Civ1 results. Choose a smaller smoothing parameter [wiki:FieldSmooth !FieldSmooth]=2, to limitate systematic smoothing effects in the final result. 
     53Fix2 and Patch2 act on the Civ2 results like Fix1 and Patch1 on the Civ1 results. Choose a smaller smoothing parameter [wiki:FieldSmooth !FieldSmooth]=2, to limitate systematic smoothing effects in the final result.
    5454
    55 The final vorticity field can be observed in the following figure, in which the  vorticity roll up in the wake of the cylinder is clearly visible. A zoom  near a vortex shows the vorticity superposed with velocity vectors. 
     55The final vorticity field can be observed in the following figure, in which the  vorticity roll up in the wake of the cylinder is clearly visible. A zoom  near a vortex shows the vorticity superposed with velocity vectors.
    5656
    57 A cut of the velocity along a transverse line x=250, y from 0 to 300 (in pixel coordinates), provides a good representation of the strong velocity shear in the wake of the cylinder. This can be done by displaying the velocity field filter2, open set_object/line, choose ! ProjMode=inter_tps, Mesh=2 to get the profile with spline interpolation from filter2. Then select hold on on the GUI view_field and repeat the same cut with the field 'civ2', '''!!ProjMode'''='projection'. We can then compare the civ2 measurement points to the interpolation, showing some fluctuations are smoothed out but without widening of the strong shear zone. The result has been exported in figure **, using the menu bar tool '''[Export/extract figure] '''in '''view_field'''.  The typical precision can be estimated from the scattering of the points as +-0.1 px, with typically 5-10 pixels in spatial resolution.
     57A cut of the velocity along a transverse line x=250, y from 0 to 300 (in pixel coordinates), provides a good representation of the strong velocity shear in the wake of the cylinder. This can be done by displaying the velocity field filter2, open set_object/line, choose ! ProjMode=inter_tps, Mesh=2 to get the profile with spline interpolation from filter2. Then select hold on on the GUI view_field and repeat the same cut with the field 'civ2', '''!!ProjMode'''='projection'. We can then compare the civ2 measurement points to the interpolation, showing some fluctuations are smoothed out but without widening of the strong shear zone. The result has been exported in figure **, using the menu bar tool '''[Export/extract figure] '''in '''view_field'''.  The typical precision can be estimated from the scattering of the points as +-0.1 px, with typically 5-10 pixels in spatial resolution.
    5858
    5959  [[Image(vort_civ3-2.jpg)]] [[Image(vort_vel_zoom.jpg)]]
    60  
     60
    6161= Other Example: =
    62 == Further Civ iterations  ==
     62== Further Civ iterations ==
    6363  The result can be improved again by performing a third civ iteration, civ3. For that purpose, select only the ’'''civ2'''’, ’'''fix2'''’ and ’'''patch2'''’ operations with the same parameters as previously. The previous result is now considered as ’'''civ1'''’, so set CIV as the subdirectory in the edit window '''[[https://servforge.legi.grenoble-inp.fr/projects/soft-uvmat/search?q=wiki%3ASubDirCiv SubDirCiv]1]'''. Select a new subdirectory name, for instance ’CIV3’ in the edit window '''[[https://servforge.legi.grenoble-inp.fr/projects/soft-uvmat/search?q=wiki%3ASubDirCiv SubDirCiv]2]'''. Further iterations could be similarly performed, but the improvement becomes negligible.